Method for imparting hydrophilicity to substrate

ABSTRACT

The present invention relates to a method for imparting hydrophilicity to a substrate whereby high hydrophilic properties and water-holding properties can be maintained for a long period of time. According to the present invention, an SiO 2  film is formed directly or through an undercoat layer on a substrate under a reduced pressure of 100 Pa or less and immediately after the SiO 2  film is formed, the SiO 2  film is treated with water. Before forming the SiO 2  film, it is also desirable that an undercoat layer consisting of a TiO 2  film, Al 2 O 3  film, Nb 2 O 5  film, a laminated film prepared by laminating the TiO 2  film on the Al 2 O 3  film, a laminated film prepared by laminating the TiO 2  film on the Nb 2 O 5  film, or a low emissivity film be formed on a substrate and the SiO 2  film be then formed on the undercoat film to serve as an SiO 2  composite film.

TECHNICAL FIELD

[0001] The present invention relates to a method for imparting hydrophilicity to a substrate whereby high hydrophilic properties and water-holding properties can be maintained for a long period of time.

BACKGROUND TECHNIQUE

[0002] Various measures such as the use of defogging mirrors or eyeglasses have been taken to prevent fog due to vapor or rain. For example, there is a method for applying a surface active agent to the surface of a glass or lens (hereinafter referred to as “substrate”), a method for applying a water absorbing agent to the surface of the substrate, a method for applying a water repellent agent to the surface of the substrate, or a method for coating the surface of the substrate with inorganic hydrophilic substance by a wet method.

[0003] In the method for applying the surface active agent to the surface of the substrate, by positioning a hydrophilic group on the surface of the substrate, a water screen is uniformly leveled without water adhering to produce a defogging effect.

[0004] The method for applying the water absorbing agent to the surface of the substrate is to produce a defogging effect by causing water adhering to the surface of the material to be absorbed by the water absorbing agent.

[0005] In the method for applying the water repellent agent to the surface of the substrate, by providing a coating such as a fluoroethylene resin whose surface tension is less on the substrate to increase the contact angle of water droplets, water adhering to the surface is repelled to produce a defogging effect.

[0006] In the method for coating the surface of the substrate with an inorganic hydrophilic substance, by providing a hydrophilic thin film with a small contact angle for the water droplets on the surface of the substrate, the water screen is made uniformly level without water adhering to produce a defogging effect, wherein a wet method such as a sol-gel method or a spin coat method is adopted as a coating process.

DISCLOSURE OF THE INVENTION

[0007] In the method for applying the surface active agent, there is a problem that the surface active agent cannot be used for a long time because it is easily swept away by water.

[0008] In the method for applying the water absorbing agent, there is a problem that the water absorbing agent cannot withstand long term use because it will soon be saturated if there is a lot of water or if an extremely thick water absorbing agent coating is required to allow the water absorbing agent to exhibit a water absorbing effect to some extent and as a result, the reflected image is distorted, or the water absorbing agent exhibits inferior flaw resistance because of the presence of an organic material.

[0009] In the method for applying the water repellent agent, there is a problem that though the water repellent agent repels water, it does not have sufficient defogging properties to repel even minute water droplet and it is difficult to maintain the defogging property for a long time.

[0010] Further, in the method for coating with an inorganic hydrophilic substance, there is a problem that uniform coating is not possible over a large area such as a mirror because it is difficult to control the coating thickness in the order of nm. It is complicated because there is a plurality of treating processes such as spraying of coating solution and burning, and it is difficult to maintain the defogging properties for a long time because the mechanical strength on the coating surface is insufficient.

[0011] To solve the above-mentioned problems, according to the present invention, an SiO₂ film is formed directly or through an undercoat layer on a substrate under a reduced pressure of 100 Pa or less and immediately after the SiO₂ film is formed, the SiO₂ film is treated with water to selectively form silanol groups (SiOH) on the surface of the SiO₂ film. The silanol groups (SiOH) formed on the SiO₂ film exhibit hydrophilic properties.

[0012] In the present invention, the term “SiO₂ film” is used to clearly express a silicon dioxide film, but this also includes a film in which the ratio of an Si atom to an O atom is not always 1:2 stoichiometrically. This also applies to an SnO₂ film.

[0013] A TiO₂ film, a Al₂O₃ film, a Nb₂O₅ film, a laminated film prepared by laminating the TiO₂ film on the Al₂O₃ film, a laminated film prepared by laminating the TiO₂ film on the Nb₂O₅ film, or a low emissivity film is desirable as the undercoat layer. In this case, it is desirable that the undercoat layer be formed on the substrate through the SiO₂ film. It is also desirable that the SnO₂ film be selected as a structure for the low emissivity film. If the SnO₂ film which is formed through a laminated film prepared by laminating the SnO₂ film and the SiO₂ film on the substrate in turn by a CVD method is selected, it is possible not only to decrease an interference reflected color of a thin film, but also to prevent seepage of alkalis (Na) from a glass substrate.

[0014] An SiN film is also effective as the undercoat layer provided to prevent seepage of the alkalis (Na). A thickness of 1˜20 nm is desirable for the SiN film.

[0015] As described above, the SiO₂ film is formed under a reduced pressure of 100 Pa or less. However, there is a method for forming the SiO₂ film by a plasma CVD under an atmosphere of 100˜1 Pa of which the main components are silane gas and oxidizing gas, a method for forming the SiO₂ film by sputtering under a reduced pressure of 10˜0.1 Pa, or a method for forming the SiO₂ film by vacuum deposition under a reduced pressure of 1 Pa or less, or the like. Above all, the method for forming the SiO₂ film by sputtering is most excellent as a method for treating a larger area. When the SiO₂ film is formed by sputtering an Si target under an oxygen atmosphere, if 1˜400% (preferably 1-200%, more preferably 60˜80%) of argon or nitrogen is mixed relative to the oxygen in an oxygen atmospheric gas, it is possible to selectively form a silanol group by providing a large oxygen deficiency on the surface of the SiO₂ film.

[0016] In the case where a glass substrate is selected for the substrate, it is also possible to obtain a hydrophilic treating surface by burning, after water treatment, the glass substrate at a temperature of 300˜500° C. under atmospheric pressure in which 0˜100% of nitrogen is provided relative to air. In this manner, it is possible to remove any organic substance adhering to the surface which would become water repellent component and to form a highly hydrophilic film.

[0017] It is also desirable that 0.1˜20% by weight of Al component be used to dope an Si component in the SiO₂ film. In this manner, adsorption of water on the silanol group is stabilized and a film with high water-holding properties can be formed.

[0018] Further, it is desirable that the substrate have transparency or mirror reflection and the transparency or the mirror reflection be maintained even after the hydrophilic treatment.

PREFERRED EMBODIMENTS FOR WORKING THE INVENTION

[0019] Preferred embodiments of the present invention will now be explained hereunder.

[0020] In a method for imparting hydrophilic properties to a substrate according to the present invention, an SiO₂ film is formed on the substrate by a film-forming method under a reduced pressure and immediately after formation thereof, the SiO₂ film is treated with water. There is no specific limit to the material for the substrate used in the present invention as far as required to provide the substrate with a defogging function, hydrophilic function or water-holding function. For example, a suitable material includes glass, a mirror, a lens (including a plastic lens), tile or an aluminum panel.

[0021] As methods for forming an SiO₂ film on the substrate under a reduced pressure of 100 Pa or less, there are a plasma CVI) method, a sputtering method, a vacuum deposition method and the like. The reason why a silanol group is formed by performing water treatment immediately after an SiO₂ film is formed under a highly reduced pressure is because water reacts before chemical adsorption of an organic substance onto the surface of chemically unstable SiO₂ film formed by a film forming method under reduced pressure, oxidation at a surface-active site, or connection of an organic substance at the active site is generated and therefore the silanol group which contributes to the hydrophilic properties is easily formed.

[0022] In particular, since the method for sputtering Si as a target in an oxygen atmosphere under a highly reduced pressure can form many active sites on the surface of the substrate, if this is combined with the water treatment immediately after the formation of the film, it is possible to form a large number of silanol groups (SiOH) with highly hydrophilic properties on the surface of the SiO₂ film.

[0023] By forming a large number of silanol groups on the surface of the SiO₂ film, the treated material can exhibit highly hydrophilic properties and water-holding properties. Since the Si of the silanol group when formed by the sputtering method is firmly coated on the surface of the substrate, it is possible to maintain the hydrophilic properties and water-holding properties for a long time.

[0024] Non-deionized water may be used in the water treatment. However, it is desirable that distilled water, pure water, or acid solution adjusted by adding an inorganic acid such as hydrochloric acid, sulfuric acid, nitric acid or phosphoric acid, or an organic acid such as acetic acid, butyric acid or lactic acid to the pure water, or an alkaline solution adjusted by adding a base such as sodium hydroxide, potassium hydroxide or ammonium hydroxide to the pure water be used. As a method for water treatment, there is a method for dipping a material formed with an SiO₂ film in water or a method for passing the material through water vapor, a method for spraying on the material, or the like.

[0025] As an atmospheric gas in the case where the SiO₂ film is formed, silane gas of 100˜1 Pa and an oxidizing gas or the oxidizing gas mixed with rare gas or hydrogen gas are used in the plasma CVD. In the vacuum deposition method, there is a method for depositing an SiO₂ source by reducing the atmospheric pressure to 0.1 Pa or less, or a method for utilizing an SiOx (X=0˜2) source under an atmosphere of gaseous oxygen of 1˜0.1 Pa or the rare gas or mixed gas thereof.

[0026] An atmospheric gas used in the sputtering method may be gaseous oxygen alone, but a method for mixing 1˜400% by volume, preferably 1˜200% by volume, more preferably 60˜80% by volume of argon gas and/or nitrogen gas relative to oxygen may be used. When the mixed gas is used, an SiOx suboxide film is formed and oxygen deficiency, i.e. a large number of active sites exist on the surface of the SiOx suboxide film. As a result, it is possible to form a large number of silanol groups.

[0027] Further, as an other means for forming a large number of silanol groups, before forming the SiO₂ film, it is also desirable that an undercoat layer consisting of a TiO₂ film, Al₂O₃ film, Nb₂O₅ film, a laminated film prepared by laminating the TiO₂ film on the Al₂O₃ film, a laminated film prepared by laminating the TiO₂ film on the Nb₂O₅ film, or a low emissivity film be formed on a substrate and the SiO₂ film be then formed on the undercoat film to serve as an SiO₂ composite film. The undercoat layer has a function of imparting adhesion between the SiO₂ film and the substrate, to improve durability of the film and to adjust the surface irregularity. By treating the composite film with water, it is possible to selectively form the silanol group.

[0028] It is also desirable that Al be mixed with Si material in advance when an SiO₂ film is formed or 0.1˜20% by weight, preferably 1˜10% by weight of an Al component be doped relative to an Si component of the SiO₂ film by the treatment after formation of the silanol group by water treatment. Since the Al component is doped, adsorption conditions of water to the silanol group are stabilized and as a result, the water-holding properties of the SiO₂ film can be improved.

[0029] In this case, it is desirable that the undercoat layer be formed on the substrate through the SiO₂ film. It is also desirable that an SnO₂ film be selected as a low emissivity film. If the SnO₂ film formed through a laminated film prepared by laminating the SnO₂ film and SiO₂ film on the substrate in turn by a CVD method is selected, it is also possible to decrease interference of reflected color of a thin film and to prevent seepage of alkali (Na) from a glass substrate.

[0030] It is also effective to form an SiN film of a thickness of 1˜20 nm as the undercoat layer for the purpose of preventing seepage of alkali (Na) from the glass substrate.

[0031] In the case where the substrate is composed of glass material, it is desirable that the substrate be burned at a temperature of 300˜500° C. after water treatment in an atmosphere in which 0˜100% of nitrogen is mixed relative to air. With this burning, it is possible to remove organic substances adhering to the surface which becomes a repellant component and to improve the hydrophilic properties.

[0032] The thickness of the SiO₂ film formed by the treating method of the present invention is adjusted to 1˜100 nm, preferably 1˜65 nm. The reason for this is that in the case of under 1 nm, it is not possible to sufficiently exhibit the hydrophilic properties and even though the thickness is over 100 nm, it is not possible to expect an increased effect.

[0033] A film with a thickness of this range formed by the sputtering method is very flat compared with the plasma CVD method or the vacuum deposition method, and the hydrophilic properties can be improved. The sputtering method also has a characteristic in that uniform hydrophilic film can be formed even on a substrate with a large area such as a mirror.

EMBODIMENTS AND COMPARATIVE EXAMPLES

[0034] The present invention will now be described with reference to specific embodiments and comparative examples.

[0035] (Embodiment 1)

[0036] A test substrate is prepared by cutting a float glass at 150 mm×150 mm. An undercoat layer of titanium oxide with a thickness of 500 nm is formed on the substrate by a sputtering method in an oxygen atmosphere.

[0037] Next, an SiO₂ film of a thickness of 10 nm is formed on the undercoat layer by the sputtering method in the oxygen atmosphere and immediately after that, it is dipped in the acid solution for water treatment to form a silanol group on the SiO₂ film.

[0038] The contact angle of a water droplet on a test panel prepared by the above method was measured. As a result, the contact angle was 2° and had highly hydrophilic properties. This test panel was left in a room for 27 days. The contact angle of the water droplet was measured again after 27 days and, as a result, the same angle (2°) as before was found, confirming that highly hydrophilic properties could be maintained for a long time. The results are shown in Table 1. A method for measuring the contact angle of a water droplet is described below. TABLE 1 Gas SiO₂ Water droplet composition film Water contact angle (°) No. Undercoat layer (O₂/Ar) thickness treatment 0 day 27th day Embodiment 1 TiO₂ (500 nm) 100/70 10 nm Acid water 2  2  2 Low emissivity film 100/70 10 nm Acid water 2  9 (400 nm)  3 Low emissivity film 100/70 CVD10 nm Acid water 2 13 (400 nm)  4 Nil 100/70 15 nm Acid water 2 20  5 Nil 100/70 65 nm Acid water 2 14  6 Nil — Wet method Acid water 3 17 100 nm   7 Nb₂O₅ (10 nm) 100/70  7 nm Acid water 2 13 TiO₂ (250 nm)  8 Nb₂O₅ (10 nm) 100/70 15 nm Acid water 2 15  9 SiN (10 nm) 100/70 45 nm Acid water 2 15 10 SiO₂ (100 nm) 100/70  5 nm Acid water 2 13 Comparative Nil 100/0  15 nm Nil 2 32 example 1  2 Only low emissivity — Nil Nil 2 34 film (400 nm)  3 Float glass of 3 mm — Nil Nil 2 38  4 TiO₂ (500 nm) — Nil Nil 2 40

[0039] Test panels were prepared in the same manner as the embodiment 1 except that the undercoat layer, gas composition, SiO₂ film thickness, and water treatment conditions of the embodiment 1 were changed as shown in Table 1. These results are shown in Table 1.

[0040] (Embodiment 10)

[0041] A test substrate is prepared by cutting a float glass at 150 mm×150 mm. This substrate was dipped in a 30% solution of H₂SiF₆ for 90 minutes to form an SiO₂ undercoat film of a thickness of 100 nm.

[0042] Next, an SiO₂ film of a thickness of 5 nm was formed on the undercoat film by a sputtering method in an oxygen atmosphere and then dipped in an acid solution for water treatment, thereby forming a silanol group on the SiO₂ film.

[0043] (Embodiment 11)

[0044] A test substrate was prepared by cutting a float glass at 150 mm×150 mm. An SiO₂ film of a thickness of 15 nm was formed on the substrate by a sputtering method in a mixed atmosphere of oxygen and argon (Q₂/Ar % by volume=100/70).

[0045] The SiO₂ film, once formed, is then dipped in an acid solution for water treatment to form a silanol group on the SiO₂ film. Treatment conditions are shown in Table 2. TABLE 2 Undercoat Gas composition SiO₂ film Water No. Substrate kind layer (O₂/Ar) thickness treatment Al dope Embodiment 11 Float glass of 3 mm Nil 100/70 15 nm Yes Nil 12 Mirror of 5 mm Nil 100/70  7 nm Yes Nil 13 Float glass of 3 mm Yes 100/70  7 nm Yes 8 wt % 14 Float glass of 5 mm Nil 100/70  5 nm Yes 8 wt % 15 Float glass of 5 mm Nil 100/70  1 nm Yes 8 wt % Comparative Float glass of 3 mm Nil 100/70 15 nm Nil 6 wt % example 5  6 Float glass of 5 mm Nil 100/40 15 nm Nil 6 wt %  7 Float glass of 3 mm Nil 100/80 15 nm Nil 6 wt %  8 Float glass of 3 mm Nil 100/60 15 nm Nil 6 wt %

[0046] The test panels prepared by the above-mentioned method were carefully maintained in a room in an inclined and fixed condition. Water was sprayed on the panels on every date shown in Table 3 and the wetting degree of the surface after 60 seconds was visually observed and evaluated. In the case where the surface is covered with dust and wettability is affected, the surface is wiped with a non-woven fabric cloth for further evaluation. Evaluation criteria are as follows: TABLE 3 Hydrophilic property evaluation No. 1^(st) day 3^(rd) day 6^(th) day 11^(th) day 16^(th) day 25^(th) day 29^(th) day 47^(th) day 51^(st) day Embodiment 11 ⊚ ⊚ ⊚ — ⊚ — ⊚ — ◯ 12 ⊚ ⊚ ⊚ — ⊚ — ⊚ — ⊚ 13 ⊚ — ⊚ ⊚ ⊚ — ⊚ — ⊚ 14 ⊚ — ⊚ ⊚ ⊚ — ⊚ — ⊚ 15 ⊚ — ⊚ ⊚ ⊚ — ◯ — ◯ Comparative ◯ ◯ Δ — ◯ — Δ — Δ example 5  6 Δ X X Stopped  7 ◯ ◯ Δ — Δ — X — Δ  8 ◯ ◯ Δ — Δ — Δ — Δ

[0047] Each test panel was prepared and evaluated in the same manner as in the embodiment 11 except that the substrate kind, the undercoat layer, the gas composition, the SiO₂ film thickness, with or without water treatment, and with or without Al dope were changed. Each condition is shown in Table 2 and evaluation results are shown in Table 3.

INDUSTRIAL APPLICABILITY

[0048] As described above, according to the present invention, immediately after an SiO₂ film is formed on the substrate under a reduced pressure of 100 Pa or less, the SiO₂ flm is treated with water to selectively form a silanol group (SiOH) on the surface of the SiO₂ film. Accordingly, it is possible to maintain highly hydrophilic properties and water-holding properties for a long time without affecting the optical function and outer appearance of the substrate itself after a defogging treatment.

[0049] Accordingly, the substrate obtained by the method of the present invention can be expected to provide a fog prevention effect for improving visibility in a high temperature and highly humid atmosphere and can be suitably applied to defogging mirror or window in a bath room or the like. Also, since the SiO₂ film formed has an extremely small contact angle with water, it has the effect of removing any fouling composition contained in the water together with a water stream without retaining the fouling composition on the substrate. Accordingly, it is possible to prevent fouling of the glass or panel.

[0050] Further, in the case where the glass substrate is used, the refraction factor between the SiO₂ film formed and the substrate is almost the same. Accordingly, there is no optical strain and transparency is also high. In the case where the sputtering method is used as a method for forming the SiO₂ film, since control of the film thickness in the order of several nm is possible, it is possible to obtain material with a larger area having a uniform SiO₂ film. 

What is claimed is:
 1. A method for imparting hydrophilicity to a substrate comprising the steps of forming an SiO₂ film directly or through an undercoat layer on the substrate under a reduced pressure of 100 Pa or less, treating the SiO₂ film with water immediately after formation of the film, and selectively forming a silanol group (SiOH) on the surface of the SiO₂ film.
 2. The method for imparting hydrophilicity to a substrate according to claim 1, wherein the undercoat layer comprises a TiO₂ film, an Al₂O₃ film, a Nb₂O₅ film, a laminated film prepared by laminating the TiO₂ film on the Al₂O₃ film, a laminated film prepared by laminating the TiO₂ film on the Nb₂O₅ film, or a low emissivity film.
 3. The method for imparting hydrophilicity to a substrate according to claim 2, wherein the low emissivity film comprises a SnO₂ film.
 4. The method for imparting hydrophilicity to a substrate according to claim 1 or claim 2, wherein the undercoat layer is formed on the substrate through the SiO₂ film.
 5. The method for imparting hydrophilicity to a substrate according to claim 3, wherein the SnO₂ film is formed on the substrate through a laminated film prepared by laminating the SnO₂ film and the SiO₂ film in turn.
 6. The method for imparting hydrophilicity to a substrate according to claim 1, wherein the undercoat layer comprises an SiN film.
 7. The method for imparting hydrophilicity to a substrate according to claim 6, wherein a thickness of the SiN film is 1˜20 nm.
 8. The method for imparting hydrophilicity to a substrate according to claims 1 through 7, wherein the reduced pressure of 100 Pa or less is characterized in that 1˜400% of argon or nitrogen is mixed relative to oxygen in oxygen atmospheric gas.
 9. The method for imparting hydrophilicity to a substrate according to claims 1 through 8, wherein the method for forming the SiO₂ film is performed by sputtering an Si target.
 10. The method for imparting hydrophilicity to a substrate according to claims 1 through 9, wherein the thickness of the SiO₂ film is 1˜100 nm, preferably 1˜65 nm.
 11. The method for imparting hydrophilicity to a substrate according to claims 1 through 10, wherein the substrate is a glass substrate which is burned after water treatment at a temperature of 300˜500° C. under an atmosphere in which 0˜100% of nitrogen is mixed relative to air.
 12. The method for imparting hydrophilicity to a substrate according to claims 1 through 11, wherein 0.1˜20% by weight of an Al component is doped relative to an Si component in the SiO₂ film.
 13. The method for imparting hydrophilicity to a substrate according to claims 1 through 12, wherein the substrate provides transparency or a mirror reflection and the transparency or mirror reflection can even be maintained by the hydrophilic treatment. 